1. Field of the Invention
The invention is directed to intermetallic powders. More particularly, methods for modifying atomized intermetallic aluminide powders and powder metallurgical methods for forming articles from the modified powders are provided.
2. Description of Related Art
Metal powders can be fabricated by atomization techniques, which form metal powder from molten metal using a spray of droplets. Elemental and pre-alloyed metal powders can be formed by atomization techniques. Water atomization and gas atomization techniques are commonly used to form these powders.
Water atomization utilizes rapid solidification of molten metal, which produces irregular shaped particles having a rough surface texture. In addition, water atomized powders can have a high oxygen and oxide level. For example, the water atomization of FeAl alloy powders can require special precautions to reduce the oxygen content of the particles and prevent the formation of oxides of iron and aluminum on the surface. Excessive oxidation of such particles can produce stringers of oxides in articles made from the particles. See M. R. Hajaligol, et al., “A Thermomechanical Process to Make Iron Aluminide (FeAl) Sheet,” Mater. Sci. Eng. A258 (1998) pp. 249-257. A non-uniform distribution of oxides can cause property variations within articles and also result in article-to-article property variations. See U.S. Pat. No. 6,030,472 to Hajaligol et al., which is hereby incorporated herein by reference in its entirety. In addition, a high surface oxide content reduces bonding between particles during sintering. See R. E. Mistler, et. al., “Tape Casting as a Fabrication Process for Iron Aluminide (FeAl) Thin Sheets,” Mater. Sci. Eng. A258 (1998) pp. 258-265.
In contrast, gas atomization techniques utilize gases, such as air, nitrogen or inert gases, as the atomizing fluid and achieve lower cooling rates of molten metal than water atomization techniques. Due to the use of these gases and lower cooling rates, powders produced by gas atomization techniques are more spherical, have less oxidation and surface oxides, and have a higher apparent density, than water atomized powders.
The packing density of atomized powders is related to the particle surface area and particle shape. Water atomized powders have irregular shapes and associated increased surface area. Gas atomized powders are typically smaller in size and more regular shaped than water atomized powders. In general, water atomized powders have a lower packing density than gas atomized powders. A lower packing density can significantly affect the final density and resulting mechanical properties of the material.
The mechanical properties of a metallic alloy can also be affected by the amount and distribution of oxides in the powder particles. Generally, well-distributed oxides can strengthen an alloy. However, oxide stringers can provide undesirable initiation sites for cracks and deleteriously affect mechanical properties of articles. In addition, oxide stringers can negatively affect the sintering performance of articles, resulting in articles having unsatisfactory microstructures and/or properties. Such oxide stringers can thus limit the performance of articles made from water atomized powders.
In contrast, gas atomized powders have a lower oxygen content and thus are less susceptible to oxide stringer problems than water atomized powders. However, due to their low oxygen content, gas atomized powders do not contain well-distributed oxides. Without such well-distributed oxides, the mechanical properties of gas atomized powders can be unsatisfactory for some applications. In addition, gas atomized powders can be unsatisfactory for some powder metallurgical techniques where they provide insufficient particle bonding in the green state.